Organic-inorganic interplay and paleoenvironmental reconstruction of Asia's largest opencast lignite mine deposits.

Asia's largest opencast lignite mines, located in Neyveli, Tamil Nadu, India, were studied to evaluate the organic and inorganic characteristics and reconstruct the paleo-depositional environment of the lignite deposits. The investigation involved advanced geochemical and analytical techniques, including X-ray fluorescence, field emission scanning electron microscopy (FE-SEM), as well as proximate and ultimate analyses. The physical properties of the lignite revealed a moisture content ranging from a minimum of 7.5% to a maximum of 25%, with an average of 14.4%. The ash content varied between 2.6 and 19%, with an average of 6.6%, suggesting the low mineral impurity levels in the lignite deposits. Volatile matter ranged from 38.2 to 48.9%, while fixed carbon content was observed between 31.4 and 40.3%. The ultimate analysis identified carbon content ranging from 51.93 to 78.58%, with moderate levels of hydrogen (< 5%), nitrogen (< 5%), and sulfur (< 1%). Variations in the Gross Calorific Value (4608-5882 kcal/kg) reflect the heterogeneous nature of the lignite, emphasizing the necessity of utilization strategies based on energy density. Major oxide analysis demonstrated strong correlations among SiO₂, Al₂O₃, and TiO₂ with ash yield, suggesting a shared origin and close association with the inorganic constituents of the lignite. Trace elements revealed significant associations among elements such as Zr, V, Sr, Nb, Ga, and Cr, providing the geochemical composition of Neyveli lignite. Microstructural investigation via FE-SEM revealed a fine-grained morphology, distinct mineral phases, and porosity characteristics. Weathering indices and alteration studies pointed to a felsic to intermediate igneous provenance for the inorganic components. The study situates the Neyveli lignite deposits within a deltaic depositional system formed during the late Cretaceous to early Tertiary periods. This paleoenvironment was characterized by dynamic hydrological regimes, as evidenced by sedimentary structures and interbedded lignite layers.